The present invention relates to a vapor phase growth of a compound semiconductor and more particularly to the vapor phase growth, by which an epitaxial layer having a high resistivity of, for example, 100,000 ohm-cm or higher, can be produced stably having a good reproducibility. The present invention is also related to a vapor phase growth of semiconductor layers having an interface, at which the distribution profile of the impurity concentration is abruptly changed.
Such semiconductor devices using the III-V group compounds, e.g., GaAs, as a field effect transistor, have conventionally been produced by a vapor phase epitaxy (VPE) of an active layer having a submicron thickness on a semi-insulating crystalline GaAs substrate. It has, however, been impossible to avoid the problem that the crystal defects, i.e., crystallographic defects such as lattice defects and dislocations, of the semi-insulating substrate extend to the active layer during the crystal growth and cause modification of electric properties of the active layer which grows on the substrate. This problem degrades the properties, especially the output and the efficiency, of the semiconductor devices. In order to overcome this problem, a buffer layer having a high resistivity is usually located between the semi-insulating crystalline substrate and the active layer, so as to exclude the influence of the substrate on the active layer.
In the VPE method employed for the growth of the compound semiconductors, a reaction chamber is provided with high- and low-temperature regions forming a temperature distribution between these regions. Gallium, which is a source material, and arsenic trichloride, which is a reaction gas, are brought into a disproportionation reaction at the high temperature region, and the product of the reaction, which is growth gas, is brought into contact with a crystalline substrate positioned at the low temperature region downstream of the gallium thereby growing a crystal on the substrate.
Compound semiconductor devices with a buffer layer and an active layer have been produced by means of the VPE process explained above. A high resistivity first epitaxial layer as the buffer layer and then a highly doped second epitaxial layer as an active layer on the crystalline substrate are continuously grown. When this continuous growth, namely a multilayer VPE, is repeated in individual reaction chambers, the impurities of a high concentration, which are introduced into the reaction chambers at the growth stage of the active layer, contaminate the wall of the reaction chamber, the source material and the like, and therefore, it becomes difficult to obtain the buffer layer having a high resistivity. This is a major reason for the low yield of the conventional multi-layer VPE.
In the production of the compound semiconductor devices having a buffer layer, the impurity concentration in the growth gas is changed from a low value for growth of a high resistivity layer to the low value for the growth of the low resistivity layer, which change however does not occur instantaneously but with a lapse of time ranging from a few tenth of a second to a few minutes.
Therefore, a considerably wide distribution of impurities at the interface between both layers is inevitably formed. The gradient of the so formed distribution of the impurities is small, with the consequence that, generally speaking, the characteristics of the semiconductor devices deteriorate, especially the noise figure.
In order to obtain a steep gradient of the concentration distribution, various methods including a method of etching a crystal by an HCl gas during the growth of the crystal until the equilibrium of the impurity concentration was reached, were tested. According to the most successful method, the grown layer thickness required for changing the carrier concentration from 10.sup.16 /cm.sup.3 to 10.sup.15 /cm.sup.3 was approximately 1800 .ANG. (18 nm) and the grown layer thickness required for changing the carrier concentration of from 10.sup.17 /cm.sup.3 to 10.sup.15 /cm.sup.3 was slightly less than 2500 .ANG. (25 nm).